Hydrogen-bonding-mediated structural stability and electrochemical performance of iron fluoride cathode materials

Abstract

Numerous lithium ion battery cathode materials containing trace amounts of water accommodated in Li⁺ transportation tunnels have been experimentally synthesized. However, the impact of water on structural stability and electrochemical performance of cathode materials is still unclear. Here, the first-principles calculations combining thermodynamic analysis of Li_xFeF₃·0.33H₂O were performed to unravel the interaction mechanism among frameworks of FeF₃, H₂O, and Li⁺. The FeF₃ framework structure distortion is mitigated by hydrogen bonding between isolated H₂O and F⁻ ions, bringing opposite effects on the stability of hydrogen bonding and instability of structural distortion. The hydrogen bonding strength of F⁻⋯H₂O can be further mediated by the Li⁺-inserted amount, which indirectly results in a wide discharge voltage window of 2.2 to 3.6 V. The Li⁺ transportation barrier in cooperative mode is also tuned by the flexible hydrogen bonding strength due to different occupied positions. Li_0.66FeF₃·0.33H₂O is determined as the most stable species and more Li⁺ insertion directly leads to the conversion reaction FeF₆³⁻ → FeF₄⁻ + 2F⁻. Therefore, stabilizing Fe–F bonds and reducing octahedral chain distortion are important to improve the electrochemical performance of FeF₃ cathode materials with water.